We had six roof trusses to build for out shelter. The logs we had to work with varied in diameter from 18 inches to about 8.
We knew we needed the trusses to span about 20 feet. We decided to have an additional 2 feet on either end for some overhang (see the reason we had for this replacement). So we needed a minimum of 24 foot long "tie beams". We also decided to have the roof peak at 5 feet above the tie beam.
We needed a "king post" in the center. And two rafters that connected the king post to the ends of the tie beam. And finally, we wanted smaller "struts" to connect about the mid-point of the rafter to the tie beam. (See the notes in the photo.)
But with all the different sizes of logs we knew we needed a few constants...
Ideally, the outer edge of the rafters should be consistent. We planned on spiking in 4x4 s for the plywood sheathing. Avoiding having to notch or shim these would make the roof much more level of a surface. In order to achieve that, we knew the length of the apex and the length of the tie beam span. It was just some trig and we knew the length for the rafters.
But we couldn't just cut the rafters to length. We needed them to be notched. And with the variable diameter of the king posts, tie beams and rafters no calculations were going to hold true from one truss to the next.